Carrier phase acquisition and tracking are common issues in communication systems. In acquisition, an initial carrier phase synchronization may be performed at a receive-end of the communication system. However, varying channel conditions, phase noise, oscillator mismatch and drifts (at transmitter and/or receiver), etc., can cause the carrier phase to vary throughout a reception, and it is often beneficial to continue to adjust the phase at the receiver (termed “phase tracking”). This may be done, e.g., on a continuous basis, on a periodic basis, or on an as-needed basis (e.g., a process may be triggered when some criterion is satisfied (e.g., error rate or some quality measure falls below a predetermined level)).
Phase tracking (and/or acquisition) may be particularly difficult in weak-signal environments, e.g., where the received signal is below a “noise floor” in the band in which it is received (i.e., has a “negative signal-to-noise ratio (SNR),” in decibels). This may be done deliberately, e.g., using direct-sequence spread-spectrum (DSSS) or other spectral spreading techniques, for example, to avoid interfering with other signals transmitted in the same band, to allow multiple access in the same band, and/or to decrease detectability of the signaling by unintended receivers. It may also occur unintentionally, e.g., due to signal attenuation, intentional or unintentional electromagnetic interference from other sources, etc. One example is the transmission of DSSS signals over satellite links, where the DSSS signals have their energy purposely spread over a wide band (wider, at least, than a bandwidth that would normally be needed to transmit the signals, were they not spread), and where the satellite link may introduce significant attenuation, e.g., where the satellite used is in geosynchronous orbit.
Synchronization in phase (and often frequency, as well) may be further complicated by the fact that baseband symbol instants are often unknown at the receiver, or may be approximated/extrapolated. In some cases, extrapolation may cause the receiver to extract “timing instants” that occur before the signal has actually arrived at the receiver, and which thus do not actually contain any signal energy. At negative SNRs, if timing instants with no signal energy are used in frequency/phase estimation, they may cause a failure in packet sync correlation. For more traditional approaches (in which extrapolation is not used), the same basic effect may occur when samples without any signal content are fed into a synchronization loop.
These effects are unavoidable unless the exact packet start instant is known. However, the start instant cannot be known at very low SNR without estimating frequency well enough to cause a correlation to succeed (or running a bank of frequency-adjusted correlators, which may be computationally prohibitive).
Typical methods of frequency/phase synchronization may use a phase detector and a phase-locked loop (PLL) connected to some sort of adjustable mixing device. The phase detector may be data-driven, such as using a unique word (UW) sync pattern, or data-independent, such as “power-of-2” estimation. However, such synchronization may be difficult when the symbol energy to noise power spectral density (referred to with the symbol Es/N0) is less than unity, or negative in terms of decibel units. In such cases a PLL cannot be used because the loop SNR will be too low to maintain lock. In fact, in such cases it is often difficult to even detect that a signal is present.
Pilot-aided acquisition strategies (i.e., using pilot tones or symbols) are often used in cases of low SNR in order to provide estimation benefits from known sources of frequency or phase information. However, using pilot information may result in a cost, in terms of data transmission efficiency (in power, bandwidth, time, duty cycle, etc.). For that reason, it may be beneficial to limit the use of pilot information as much as possible.